Compressed timing indicators for media samples

ABSTRACT

A first frame of data is encoded and a first timestamp associated with the first frame of data is generated. The first timestamp includes complete timing information. The first frame of data and the associated first timestamp is transmitted to a destination. A second frame of data is encoded and a second timestamp associated with the second frame of data is generated. The second timestamp includes a portion of the complete timing information. The second frame of data and the associated second timestamp is then transmitted to the destination. Additional frames of data are encoded and additional timestamps associated with the additional frames of data are generated. The majority of the additional timestamps include a portion of the complete timing information.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/903,369, filed Sep. 21, 2007, which is a continuation of U.S. patentapplication Ser. No. 11/379,025, filed Apr. 17, 2006 (now U.S. Pat. No.7,633,551), which is a continuation of U.S. patent application Ser. No.11/029,159, filed Jan. 4, 2005 (now U.S. Pat. No. 7,242,437), which is acontinuation of U.S. patent application Ser. No. 09/982,128, filed Oct.18, 2001 (now abandoned), which claims the benefit of U.S. ProvisionalApplication No. 60/241,407, filed Oct. 18, 2000, the disclosures ofwhich are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to video processing systems and, moreparticularly, to the compression of timing indicators associated withmedia samples.

BACKGROUND

The concept of recording and using timing information is fundamental to,the needs of multimedia applications. Pictures, video, text, graphics,and sound need to be recorded with some understanding of the timeassociated with- each sample of the media stream. This is useful forsynchronizing different multimedia streams with each other, for carryinginformation to preserve the original timing of the media when playing amedia stream, for identifying specific locations within a media stream,and for recording the time associated with the media samples to create ascientific or historical record. For example, if audio and video arerecorded together but handled as separate streams of media data, thentiming information is necessary to coordinate the synchronization ofthese two (or more) streams.

Typically, a media stream (such as a recorded audio track or recordedvideo or film shot) is represented as a sequence of media samples, eachof which is associated (implicitly or explicitly) with timinginformation. A good example of this is video and motion picture filmrecording, which is typically created as a sequence of pictures, orframes, each of which represents the camera view for a particular shortinterval of time (e.g., typically 1/24 seconds for each frame of motionpicture film). When this sequence of pictures is played back at the samenumber of frames per second (known as the “frame rate”) as used in therecording process, an illusion of natural movement of the objectsdepicted in the scene can be created for the viewer.

Similarly, sound is often recorded by regularly sampling an audiowaveform to create a sequence of digital samples (for example, using48,000 samples per second) and grouping sets of these samples intoprocessing units called frames (e.g., 64 samples per frame) for furtherprocessing such as digital compression encoding or packet-networktransmission (such as Internet transmission). A receiver of the audiodata will-then reassemble-the frames of audio that it has received,decode them, and convert the resulting sequence of digital samples backinto sound using electro-acoustic technology.

FIG. 1 illustrates a conventional system 100 for processing anddistributing video content. The video content is captured using a videocamera 102 (or any other video capture device) that transfers thecaptured video content onto video tape or another storage medium. Later,the captured video content may be edited using a video editor 104. Avideo encoder 106 encodes the video content to reduce the storage spacerequired for the video content or to reduce the transmission bandwidthrequired to transmit the video content. Various encoding techniques maybe used to compress the video content, such as the MPEG-2 (MovingPicture Experts Group 2nd generation) compression format.

The encoded video content is provided to a transmitter-108, whichtransmits the encoded video content to one or more receivers 110 acrossa communication link 112. Communication link 112 may be, for example, aphysical cable, a satellite link, a terrestrial broadcast, an Internetconnection, a physical medium (such as a digital versatile disc (DVD))or a combination thereof. A video decoder 114 decodes the signalreceived by receiver 110 using an appropriate decoding technique. Thedecoded video content is then displayed on a video display 116, such asa television or a computer monitor. Receiver 110 may be a separatecomponent (such as a set top box) or may be integrated into videodisplay 116. Similarly, video decoder 114 may be a separate component ormay be integrated into the receiver 110 or the video display 116.

Proper recording and control of timing information is needed tocoordinate multiple streams of media samples, such as for synchronizingvideo and associated audio content. Even the use of media which does notexhibit a natural progression of samples through time will often requirethe use of timing information in a multimedia system. For example, if astationary picture (such as a photograph, painting, or document) is tobe displayed along with some audio (such as an explanatory descriptionof the content or history of the picture), then the timing of thedisplay of the stationary picture (an entity which consists of only oneframe or sample in time) may need to be coordinated with the timing ofthe associated audio track.

Other examples of the usefulness of such timing information includebeing able to record the date or time of day at which a photograph wastaken, or being able to specify editing or viewing points within mediastreams (e.g., five minutes after the camera started rolling).

In each of the above cases, a sample or group of samples in time of amedia stream can be identified as a frame, or fundamental processingunit. If a frame consists of more than one sample in time, then aconvention can be established in which the timing informationrepresented for a frame corresponds to the time of some reference pointin the frame such as the time of the first, last or middle sample.

In some cases, a frame can be further subdivided into even smallerprocessing units, which can be called fields. One example of this is inthe use of interlaced-scan video, in which the sampling of alternatinglines in a picture are separated so that half of the lines of eachpicture are sampled as one field at one instant in time, and the otherhalf of the lines of the picture are then sampled as a second field ashort time later. For example, lines 1, 3, 5, etc. may be sampled-as onefield of picture, and then lines 0, 2, 4, etc. of the picture may besampled as the second field a short time later (for example 1/50th of asecond later). In such interlaced-scan video, each frame can betypically separated into two fields.

Similarly, one could view a grouping of 64 samples of an audio waveformfor purposes of data compression or packet-network transmission to be aframe, and each group of eight samples within that frame to be a field.In this example, there would be eight fields in each frame, eachcontaining eight samples.

In some methods of using sampled media streams that are well known inthe art, frames or fields may consist of overlapping sets of samples ortransformations of overlapping sets of samples. Two examples of thisbehavior are the use of lapped orthogonal transforms [1) HenriqueSarmento Malvar, Signal Processing with Lapped Transforms, Boston,Mass., Artech House, 1992; 2) H. S. Malvar and D. H. Staelin, “The LOT:transform coding without blocking effects,” IEEE Transactions onAcoustics, Speech, and Signal Processing, vol. 37, pp. 553-559, April1989; 3) H. S. Malvar, Method and system for adapting a digitized signalprocessing system for block processing with minimal blocking artifacts,U.S. Pat. No. 4,754,492, June 1988.] and audio redundancy coding [1) J.C. Bolot, H. Crepin, A. Vega-Garcia: “Analysis of Audio Packet Loss inthe Internet”, Proceedings of the 5th International Workshop on Networkand Operating System Support for Digital Audio and Video, pp. 163-174,Durham, April 1995; 2) C. Perkins, I. Kouvelas, O. Hodson, V. Hardman,M. Handley, J. C. Bolot, A. Vega-Garcia, S. Fosse-Parisis: “RTP Payloadfor Redundant Audio Data”, Internet Engineering Task Force Request forComments RFC2198, 1997.]. Even in such cases it is still possible toestablish a convention by which a time is associated with a frame orfield of samples.

In some cases, the sampling pattern will be very regular in time, suchas in typical audio processing in which all samples are created atrigidly-stepped times controlled by a precise clock signal. In othercases, however, the time between adjacent samples in a sequence maydiffer from location to location in the sequence.

One example of such behavior is when sending audio over a packet networkwith packet losses, which may result in some frames not being receivedby the decoder while other frames should be played for use with theiroriginal relative timing. Another example of such behavior is inlow-bit-rate videoconferencing, in which the number of frames sent persecond is often varied depending on the amount of motion in the scene(since small changes take less data to send than large changes, and theoverall channel data rate in bits per second is normally fixed).

If the underlying sampling structure is such that there is understood tobe a basic frame or field processing unit sampling rate (although someprocessing units may be skipped), then it is useful to be able toidentify a processing unit as a distinct counting unit in the timerepresentation. If this is incorporated into the design, the occurrenceof a skipped processing unit may be recognized by a missing value of thecounting unit (e.g., if the processing unit count proceeds as 1, 2, 3,4, 6, 7, 8, 9, . . . , then it is apparent that count number 5 ismissing).

If the underlying sampling structure is such that the sampling is soirregular that there is no basic processing unit sampling rate, thenwhat is needed is simply a good representation of true time for eachprocessing unit. Normally however, in such a case there should at leastbe a common time clock against which the location of the processing unitcan be referenced.

In either case (with regular or irregular sampling times), it is usefulfor a multimedia system to record and use timing information for thesamples or frames or fields of each processing unit of the mediacontent.

Different types of media may require different sampling rates. If timinginformation is always stored with the same precision, a certain amountof rounding error may be introduced by the method used for representingtime. It is desirable for the recorded time associated with each sampleto be represented precisely in the system with little or no suchrounding error. For example, if a media stream operates at 30,000/1001frames per second (the typical frame rate of North American standardNTSC broadcast video—approximately 29.97 frames per second) and theprecision of the time values used in the system is to one part in 10⁻⁶seconds, then although the time values may be very precise in humanterms, it may appear to processing elements within the system that theprecisely-regular sample timing (e.g. 1001/30,000 seconds per sample) isnot precisely regular (e.g. 33,366 clock increment counts betweensamples, followed by 33,367 increments, then 33,367 increments, and then33,366 increments again). This can cause difficulties in determining howto properly handle the media samples in the system.

Another problem in finding a method to represent time is that therepresentation may “drift” with respect to true time as would bemeasured by a perfectly ideal “wall clock”. For example, if the systemuses a precisely-regular sample timing of 1001/30,000 seconds per sampleand all samples are represented with incremental time intervals being33,367 increments between samples, the overall time used for a longsequence of such samples will be somewhat longer than the true timeinterval—a total of about one frame time per day and accumulating morethan five minutes of error after a year of duration.

Thus, “drift” is defined as any error in a timecode representation ofsampling times that would (if uncorrected) tend to increase in magnitudeas the sequence of samples progresses.

One example of a method of representing timing information is found inthe SMPTE 12M design [Society of Motion Picture and TelevisionEngineers, Recommended Practice 12M: 1999] (hereinafter called “SMPTEtimecode”). SMPTE timecodes are typically used for television video datawith timing specified in the United States by the National TelevisionStandards Committee (NTSC) television transmission format, or in Europe,by the Phase Alternating Line (PAL) television transmission format.

SMPTE timecode is a synchronization signaling method originallydeveloped for use in the television and motion picture industry to dealwith video tape technology. The challenge originally faced withvideotape was that there was no “frame accurate” way to synchronizedevices for video or sound-track editing. A number of methods wereemployed in the early days, but because of the inherent slippage andstretching properties of tape, frame accurate synchronization met withlimited success. The introduction of SMPTE timecode provided this frameaccuracy and incorporated additional functionality. Additional sourceson SMPTE include “The Time Code Handbook” by Cipher Digital Inc. whichprovides a complete treatment of the subject, as well as an appendixcontaining ANSI Standard SMPTE 12M-1986. Additionally, a text entitled“The Sound Reinforcement Handbook” by Gary Davis and Ralph Jones forYamaha contains a section on timecode theory and applications.

The chief purpose of SMPTE timecode is to synchronize various pieces ofequipment. The timecode signal is formatted to provide a system wideclock that is referenced by everything else. The signal is usuallyencoded directly with the video signal or is distributed via standardaudio equipment. Although SMPTE timecode uses many references from videoterminology, it may also be used for audio-only applications.

In many applications, a timecode source provides the signal while therest of the devices in the system synchronize to it and follow along.The source can be a dedicated timecode generator, or it can be (andoften is) a piece of the production equipment that provides timecode inaddition to its primary function. An example of this is a multi-trackaudio tape deck that provides timecode on one track and sound for theproduction on other tracks. Video tape often makes similar use of a cuetrack or one of its audio sound tracks to record and play back timecode.

In other applications, namely video, the equipment uses timecodeinternally to synchronize multiple timecode sources into one. An examplewould be a video editor that synchronizes with timecode from a number ofprerecorded scenes. As each scene is combined with the others to makethe final product, their respective timecodes are synchronized with newtimecode being recorded to the final product.

SMPTE timecode provides a unique address for each frame of a videosignal. This address is an eight digit number, based on the 24 hourclock and the video frame rate, representing Hours, Minutes, Seconds andFrames in the following format:

-   -   HH:MM:SS:FF

The values of these fields range from 00 to 23 for HH, 00 to 59 for MM,00 to 59 for SS, and 00 to 24 or 29 for FF (where 24 is the maximum forPAL 25 frame per second video and 29 is the maximum for NTSC 30,000/1001frame per second video). By convention, the first frame of a day isconsidered to be marked as 00:00:00:01 and the last is 00:00:00:00 (oneframe past the frame marked 23:59:59:24 for PAL and 23:59:59:29 forNTSC). This format represents a nominal clock time, the nominal durationof scene or program material and makes approximate time calculationseasy and direct.

The frame is the smallest unit of measure within SMPTE timecode and is adirect reference to the individual “picture” of film or video. The framerate is the number of times per second that pictures are displayed toprovide a rendition of motion. There are two standard frame rates(frames/sec) that typically use SMPTE timecode: 25 frames per second and30,000/1001 frames per second (approximately 29.97 frames per second).The 25 frame per second rate is based on European video, also known asSMPTE EBU (PAL/SECAM color and b&w). The 30,000/1001 frame per secondrate (sometimes loosely referred to as 30 frame per second) is based onU.S. NTSC color video broadcasting. Within the 29.97 frame per seconduse, there are two methods of using SMPTE timecode that are commonlyused: “Non-Drop” and “Drop Frame”.

A frame counter advances one count for every frame of film or video,allowing the user to time events down to 1/25th, or 1001/30,000th of asecond.

SMPTE timecode is also sometimes used for a frame rate of exactly 30frames per second. However, the user must take care to distinguish thisuse from the slightly slower 30,000/1001 frames per second rate of U.S.NTSC color broadcast video. (The adjustment factor of 1000/1001originates from the method by which television signals were adjusted toprovide compatibility between modern color video and the previous designfor broadcast of monochrome video at 30 frames per second.)

Thus, the SMPTE timecode consists of the recording of an integer numberfor each of the following parameters for a video picture: Hours,Minutes, Seconds, and Frames. Each increment of the frame counter isunderstood to represent an increment of time of 1001/30,000 seconds inthe NTSC system and 1/25 seconds in the PAL system.

However, since the number of frames per second in the NTSC system(30,000/1001) is not an integer, there is a problem of drift between theSMPTE 12M timecode representation of time and true “wall clock” time.This drift can be greatly reduced by a special frame counting methodknown as SMPTE “drop frame” counting. Without SMPTE drop frame counting,the drift between the SMPTE timecode's values of Hours, Minutes, andSeconds and the value measured by a true “wall clock” will accumulatemore than 86 seconds of error per day. When using SMPTE drop framecounting, the drift accumulation magnitude can be reduced by about afactor of about 1,000 (although the drift is still not entirelyeliminated and the remaining drift is still more than two frame samplingperiods).

The SMPTE timecode has been widely used in the video production industry(for example, it is incorporated into the design of many video taperecorders). It is therefore very useful if any general media timecodedesign is maximally compatible with this SMPTE timecode. If suchcompatibility can be achieved, this will enable equipment designed forthe media timecode to work well with other equipment designedspecifically to use the SMPTE timecode.

Within this document, the following terminology is used. A timecodedescribes the data-used for representing the time associated with amedia sample, frame, or field. It is useful to separate the data of atimecode into two distinct types: the timebase and the timestamp. Thetimestamp includes the information that is used to represent the timingfor a specific processing unit (a sample, frame, or field). The timebasecontains the information that establishes the basis of the measurementsunits used in the timestamp. In other words, the timebase is theinformation necessary to properly interpret the timestamps. The timebasefor a media stream normally remains the same for the entire sequence ofsamples, or at least for a very large set of samples.

For example, we may interpret the SMPTE timecode as having a timebasethat consists of:

-   -   Knowledge of (or an indication of) whether the system is NTSC or        PAL, and    -   Knowledge of (or an indication of) whether or not the system        uses SMPTE “drop frame” counting in order to partially        compensate for drift.

Given this, the timestamps then consist of the representations of theparameters Hours, Minutes, Seconds, and Frames for each particular videoframe.

Many existing systems transmit all parameters of the timestamp with eachframe. Since many of the parameters (e.g., hours and minutes) do nottypically change from one frame to the next, transmitting all parametersof the timestamp with each frame results in the transmission of asignificant amount of redundant data. This transmission of redundantdata results in the transmission of more data than is necessary tocommunicate the current timing information.

The systems and methods described herein provide for the communicationof timing indicators that convey timing information using a reducedamount of data.

SUMMARY

The systems and methods described herein provide for two different typesof timestamps to be transmitted along with frames of data. A fulltimestamp includes complete timing information, such as hourinformation, minute information, second information, and a frame number.A compressed timestamp includes a portion of the complete timinginformation, such as the frame number.

When a receiving device receives a compressed timestamp, the receivingdevice maintains the previous values of the timing parameters that arenot contained in the compressed timestamp. Since the most of theinformation in a full timestamp is redundant from one frame to the next,sending a significant number of compressed timestamps between fulltimestamps reduces the amount of data that is transmitted, but does notresult in a loss of timing information.

In one embodiment, a first frame of data is encoded. A first timestampis generated and associated with the first frame of data. The firsttimestamp includes complete timing information. The first frame of dataand the associated first timestamp is then transmitted to a destination.A second frame of data is encoded and a second timestamp associated withthe second frame of data is generated. The second timestamp includes aportion of the complete timing information. The second frame of data andthe associated second timestamp is transmitted to the destination.

In another embodiment, multimedia content to be encoded is identified.The identified multimedia content is encoded into multiple frames ofdata. Full timestamps are generated and associated with a portion of theframes of data. Each full timestamp contains complete time information.Compressed timestamps are generated and associated with frames of datathat are not associated with a full timestamp. Each compressed timestampcontains a portion of the complete time information.

In a described embodiment, the full timestamps include hour information,minute information, second information, and a frame number.

In a particular implementation, the compressed timestamps include aframe number.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional system for processing and distributingvideo content.

FIG. 2 illustrates an example multimedia encoding system and an examplemultimedia decoding system.

FIG. 3 is a flow diagram illustrating a procedure for encodingmultimedia content and transmitting timestamps and associated multimediacontent frames.

FIG. 4 is a flow diagram illustrating a procedure for decodingmultimedia content that includes multiple time stamps and associatedcontent frames.

FIG. 5 illustrates an example of a suitable operating environment inwhich the systems and methods described herein may be implemented.

DETAILED DESCRIPTION

The systems and methods described herein utilize different types oftiming indicators (referred to as timestamps) to communicate timinginformation along with frames of data. The use of both full timestampsand compressed timestamps reduces the amount of timing information thatmust be communicated with the frames of data. A full timestamp includesall timing information and is sent occasionally (e.g., a few times eachsecond or once every X frames of data). Between full timestamps, aseries of compressed timestamps are communicated with the frames ofdata. The compressed timestamps contain a subset of the complete timinginformation contained in the full timestamps. The compressed timestampcontains the timing information that has changed since the last fulltimestamp was sent.

FIG. 2 illustrates an example multimedia encoding system and an examplemultimedia decoding system. A multimedia content source 202 providesmultimedia content (e.g., audio content, video content, or combinedaudio and video content) to an encoder 204. Multimedia content sourcemay be, for example, a video camera, microphone or other capture device,or a storage device that stores previously captured multimedia content.Encoder 204 includes a clock 206 and a frame counter 208. Clock 206 isused to determine timestamp information and synchronize operation ofencoder 204. Frame counter 208 keeps track of consecutive frame numbersassociated with frames of data. Encoder 204 also includes an encodingengine 210, which encodes multimedia content and other data (such astimestamp information) into multiple frames. The output of encoder 204is communicated to a transmitter 212, which transmits the encodedcontent to one or more receivers. Alternatively, transmitter 212 may bea storage device that stores the encoded content (e.g., on a DVD,magnetic tape, or other storage device).

Receiver 220 receives an encoded signal including one or more frames andcommunicates the received signal to a decoder 222. Alternatively,receiver 220 may be a device (such as a audio player and/or a videoplayer) capable of reading stored encoded content (e.g., stored on a DVDor other storage device). Decoder 222 includes a clock 224 and a counter226. Clock 224 aids in synchronizing decoder 222. Counter 226 is used toassign frame identifiers to received frames of data. Decoder 222 alsoincludes a decoding engine 228 which decodes the received signal. Afterdecoding the received signal, decoder 222 communicates the decodedcontent to a multimedia player 230 which renders the multimedia contentdefined by the decoded signal. Multimedia player may be an audio player(e.g., a CD player), a video player (e.g., a DVD player), or acombination audio player and video player. Decoder 222 may be a separatedevice or may be incorporated into another device, such as a televisionor a DVD player.

FIG. 3 is a flow diagram illustrating a procedure 300 for encodingmultimedia content and transmitting timestamps and associated multimediacontent frames. Initially, procedure 300 identifies a number of basicunits per second in a reference clock (block 302), which is representedby a parameter labeled “base_ups”. In a particular example, thereference clock has 30,000 basic units per second (also referred to as30,000 hertz). The procedure then identifies a number of basic units ofthe reference clock per media sample period (block 304), which isrepresented by a parameter labeled “base_upp”. In a particular example,each increment of a counter (such as a frame counter) occurs after 1001increments of the reference clock. In this example, if the counteradvances by five, the reference clock advances by 5005. This examplereduces the amount of data that needs to be communicated regarding theclock (i.e., sending “5” instead of “5005”).

The procedure 300 then identifies a counting type that defines themanner in which samples (or frames) are counted (block 306). Additionaldetails regarding the various counting types are provided below. Atblock 308, the base_ups, base_upp, and counting type data is transmittedto one or more receivers. These data values allow each receiver tounderstand and properly decode subsequent frames of data.

Next, the procedure receives multimedia content to be encoded andcreates a first content frame (block 310). The first content frame iscreated by encoding a portion of the received multimedia content. Theprocedure then transmits a full timestamp along with the first contentframe (block 312). The full timestamp may be embedded within the firstcontent frame or transmitted separately, but along with the fulltimestamp. The full timestamp includes the hour, minutes, seconds, andframe number associated with the first content frame.

The procedure then creates the next content frame by encoding the nextportion of the received multimedia content (block 314). At block 316,the procedure determines whether to transmit a full timestamp or acompressed timestamp. As mentioned above, a full timestamp includes alltime-related information (i.e., the hour, minute, second, and framenumber associated with the first content frame). The compressedtimestamp includes a subset of the information required for a fulltimestamp. In a particular implementation, the compressed timestampcontains the information that has changed since the last timestamp(either full or compressed) was transmitted to the receivers. Typically,the compressed timestamp includes the frame number associated with thecurrent content frame being transmitted. The compressed timestampreduces the amount of data that must be transmitted when compared withthe full timestamp. In a particular implementation, the full timestampis sent several times each second. In an alternate implementation, thefull timestamp is sent every X frames, where X is approximately 15.

In another implementation, the decision of whether to send a fulltimestamp or compressed timestamp is adjusted dynamically based on anestimate of the reliability of the communication link betweentransmitter and receiver. If the estimated reliability of thecommunication link is high, then full timestamps may be sent lessfrequently. However, if the communication link is not expected to bereliable, the full timestamps are sent more frequently.

If the procedure determines that a full timestamp should be transmitted,a full timestamp is transmitted along with the next content frame (block318). Otherwise, a compressed timestamp is transmitted along with thenext content frame (block 320). The procedure continues by returning toblock 314 to create the next content frame and determine whether a fulltimestamp or a compressed timestamp is to be transmitted along with -thenext content frame.

In a particular embodiment, the data that specifies the timebase and thestarting timestamp of a sequence of data samples (or frames) is sentusing the following pseudo-code:

send (base_ups) // unsigned integer send (base_upp) // unsigned integersend (counting_type) // defined in Table 1 send(full_timestamp_sequence_flag) // boolean send (discontinuity_flag) //boolean send (count_dropped) // boolean send (frames_value) // integerif (counting_type != ‘000’    send (offset_value) // integer send(seconds_value) // integer send (minutes_value) // integer send(hours_value) // integerThese data specify the time of the first sample of a sequence of framesand specify the timebase necessary for interpretation of the parametersof each individual timestamp. Since these data specify both the timebaseand the initial timestamp for an entire sequence of frames, they arereferred to herein as the sequence header information for thisparticular embodiment. In one embodiment, a full timestamp is includedin each sequence header. Alternatively, the sequence headers may notcontain a full timestamp. Instead, the data contained in a fulltimestamp is retrieved from the full timestamp associated with the firstframe of data following the sequence header.

The base_ups, base_upp, and counting type parameters are discussedabove. Table 1 below defines the various counting_type values.

TABLE 1 Value Meaning 000 No dropping of frames_value count values andno use of offset_value 001 No dropping of frames_value count values 010Dropping of individual zero values of frames_value count 011 Dropping ofindividual max_pps values of frames_value count 100 Dropping of the twolowest (values 0 and 1) frames_value counts when seconds_value is zeroand minutes_value is not an integer multiple of ten 101 Dropping ofunspecified individual frames_value count values 110 Dropping ofunspecified numbers of unspecified frames_value count values 111ReservedParticular parameters are defined as follows:

-   -   full_timestamp_sequence_flag: Indicates whether every timestamp        in the following sequence of timestamps shall be fully specified        or whether some timestamps (referred to as compressed        timestamps) may only contain partial information (depending on        memory of values sent previously in the sequence header or in a        frame timestamp). If full_timestamp_sequence flag is “1”, then        full_timestamp_flag must be “1” in the timestamp information for        every frame in the following sequence.    -   discontinuity_flag: Indicates whether the time difference that        can be calculated between the starting time of the sequence and        the time indicated for the last previous transmitted frame can        be interpreted as a true time difference. Shall be “1” if no        previous frame has been transmitted.    -   count_dropped: Indicates, if discontinuity_flag is “0”, whether        some value of frames_value was skipped after the last previous        transmitted frame to reduce drift between the time passage        indicated in the seconds_value, minutes_value, and hours_value        parameters and those of a true clock.    -   frames_value, offset_value, seconds_value, minutes_value, and        hours value: Indicate the parameters to be used in calculating        an equivalent timestamp for the first frame in the sequence.        Shall be equal to the corresponding values of these parameters        in the header of the first frame after the sequence header, if        present in the sequence header.        In this embodiment, an extra signed-integer parameter called        offset_value is used in addition to the unsigned integer        frames_value, seconds_value, minutes_value, and hours_value        parameters that are used by the SMPTE timecode's timestamp, in        order to relate the time of a sample precisely relative to true        time, as shown in a formula below.

In a particular embodiment, the timestamp structure sending process forthe timestamps on individual media samples (or frames) is implementedusing the following pseudo-code:

send (full_timestamp_flag)  // boolean send (frames_value)  // unsignedinteger (if (counting_type != ‘000’) {   if (full_timestamp_flag)   send (offset_value)  // signed integer   else {    send(offset_value_flag)  // boolean    if (offset_value_flag)      send(offset_value)  // signed integer   }   if (counting_type != ‘001’)   send (count_dropped_flag)  // boolean } if (full_timestamp_flag) {  send (seconds_value)  // unsigned integer 0..59   send (minutes_value) // unsigned integer 0..59   send (hours_value)  // unsigned integer} else {   send (seconds_flag)  // boolean   if (seconds_flag) {    send(seconds_value)  // unsigned integer 0..59    send (minutes_flag)  //boolean    if (minutes_flag) {      send (minutes_value)  // unsignedinteger 0..59      send (hours_flag)  // boolean      if (hours_flag)      send (hours_value)  // unsigned integer    }   } }If any timestamp is incomplete (i.e., full_timestamp_flag is zero and atleast one of seconds_flag, minutes_flag, hours_flag, andoffset_value_flag is present and zero) the last prior sent value foreach missing parameter is used. An equivalent time specifying the timeof a media sample (in units of seconds) may be computed as follows:equivalent_time=60×(60×hours_value+minutes_value)+seconds_value+(base_upp×frames_value+offset_value)/base_upsUsing the timebase parameters, a derived parameter is defined as:max_pps=ceil(base_ups/base_upp)where ceil (x) is defined as the function of an argument x, which, fornon-negative values of x, is equal to x if x is an integer and isotherwise equal to the smallest integer greater than x. The value offrames_value should not exceed max_pps.

If count_dropped_flag is ‘1’, then:

-   -   if counting_type is ‘010’, frames_value shall be ‘1’ and the        value of frames_value for the last previous transmitted frame        shall not be equal to ‘0’ unless a sequence header is present        between the two frames with discontinuity_flag equal to ‘1’.    -   if counting_type is ‘011’, frames_value shall be ‘0’ and the        value of frames_value for the last previous transmitted frame        shall not be equal to max_pps unless a sequence header is        present between the two frames with discontinuity_flag equal to        ‘1’.    -   if counting_type is ‘100’, frames_value shall be ‘2’ and the        seconds value shall be zero and minutes value shall not be an        integer multiple of ten and frames_value for the last previous        transmitted frame shall not be equal to ‘0’ or ‘1’ unless a        sequence header is present between the two frames with        discontinuity_flag equal to ‘1’.    -   if counting_type is ‘101’ or ‘110’, frames_value shall not be        equal to one plus the value of frames_value for the last        previous transmitted frame modulo max_pps unless a sequence        header is present between the two frame with discontinuity_flag        equal to ‘1’.

As the degree of precision for the various parameters of each mediasample timestamp becomes coarser, the inclusion of the furtherinformation needed to place the timestamp within the more global scaleis optional. Any coarse-level context information that is not sent isimplied to have the same value as the last transmitted parameter of thesame type. The finely-detailed information necessary to locate theprecise sample time relative to that of neighboring samples is includedwith every timestamp, but as the degree of coarseness of the timespecification becomes higher, the inclusion of further more coarsecontext information is optional in order to reduce the average amount ofinformation that is required to be communicated.

FIG. 4 is a flow diagram illustrating a procedure 400 for decodingmultimedia content that includes multiple time stamps and associatedcontent frames. At block 402, the procedure receives base_ups, base_upp,and counting type data associated with a multimedia stream from atransmitting device. This information allows the receiving system toproperly interpret and decode the subsequently received content. Next, afull timestamp and an associated first multimedia content frame arereceived (block 404). The full timestamp provides the hours, minutes,seconds, and frame number associated with the first received frame, and,in a particular embodiment, a time offset number allowing a drift-freeprecise relation to be determined between the time computed from theother parameters and the true time of the sample.

The procedure 400 then receives a next multimedia content frame and anassociated timestamp. An associated flag (full_timestamp_flag) willindicate whether the timestamp is a full timestamp or a compressedtimestamp. The procedure decodes the multimedia content frame (block408) and determines (based on the full_timestamp_flag) whether thetimestamp is a full timestamp or a compressed timestamp (block 410). Ifthe timestamp is a full timestamp, the procedure updates all timingparameters provided by the full timestamp (block 412). If the timestampis a compressed timestamp, the procedure updates the frame parameter(block 414). The system uses the values from the most recent fulltimestamp for all other timing parameter values. Alternatively, if acompressed timestamp is received, the procedure updates all timingparameters contained in the compressed timestamp.

After updating one or more timing parameters, the procedure returns toblock 406 to receive and process the next multimedia content frame andassociated timestamp.

FIG. 5 illustrates an example of a suitable computing environment 500within which the video encoding and decoding procedures may beimplemented (either fully or partially). The computing environment 500may be utilized in the computer and network architectures describedherein.

The exemplary computing environment 500 is only one example of acomputing environment and is not intended to suggest any limitation asto the scope of use or functionality of the computer and networkarchitectures. Neither should the computing environment 500 beinterpreted as having any dependency or requirement relating to any oneor combination of components illustrated in the exemplary computingenvironment 500.

The video encoding and decoding systems and methods described herein maybe implemented with numerous other general purpose or special purposecomputing system environments or configurations. Examples of well knowncomputing systems, environments, and/or configurations that may besuitable for use include, but are not limited to, personal computers,server computers, multiprocessor systems, microprocessor-based systems,network PCs, minicomputers, mainframe computers, distributed computingenvironments that include any of the above systems or devices, and soon. Compact or subset versions may also be implemented in clients oflimited resources.

The computing environment 500 includes a general-purpose computingdevice in the form of a computer 502. The components of computer 502 caninclude, by are not limited to, one or more processors or processingunits 504, a system memory 506, and a system bus 508 that couplesvarious system components including the processor 504 to the systemmemory 506.

The system bus 508 represents one or more of several possible types ofbus structures, including a memory bus or memory controller, aperipheral bus, an accelerated graphics port, and a processor or localbus using any of a variety of bus architectures. By way of example, sucharchitectures can include an Industry Standard Architecture (ISA) bus, aMicro Channel Architecture (MCA) bus, an Enhanced ISA (EISA) bus, aVideo Electronics Standards Association (VESA) local bus, and aPeripheral Component Interconnects (PCI) bus also known as a Mezzaninebus.

Computer 502 typically includes a variety of computer readable media.Such media can be any available media that is accessible by computer 502and includes both volatile and non-volatile media, removable andnon-removable media.

The system memory 506 includes computer readable media in the form ofvolatile memory, such as random access memory (RAM) 510; and/ornon-volatile memory, such as read only memory (ROM) 512. A basicinput/output system (BIOS) 514, containing the basic routines that helpto transfer information between elements within computer 502, such asduring start-up, is stored in ROM 512. RAM 510 typically contains dataand/or program modules that are immediately accessible to and/orpresently operated on by the processing unit 504.

Computer 502 may also include other removable/non-removable,volatile/non-volatile computer storage media. By way of -example, FIG. 5illustrates a hard disk drive 516 for reading from and writing to anon-removable, non-volatile magnetic media (not shown), a magnetic diskdrive 518 for reading from and writing to a removable, non-volatilemagnetic disk 520 (e.g., a “floppy disk”), and an optical disk drive 522for reading from and/or writing to a removable, non-volatile opticaldisk 524 such as a CD-ROM, DVD-ROM, or other optical media. The harddisk drive 516, magnetic disk drive 518, and optical disk drive 522 areeach connected to the system bus 508 by one or more data mediainterfaces 526. Alternatively, the hard disk drive 516, magnetic diskdrive 518, and optical disk drive 522 can be connected to the system bus508 by one or more interfaces (not shown).

The disk drives and their associated computer-readable media providenon-volatile storage of computer readable instructions, data structures,program modules, and other data for computer 502. Although the exampleillustrates a hard disk 516, a removable magnetic disk 520, and aremovable optical disk 524, it is to be appreciated that other types ofcomputer readable media which can store data that is accessible by acomputer, such as magnetic cassettes or other magnetic storage devices,flash memory cards, CD-ROM, digital versatile disks (DVD) or otheroptical storage, random access memories (RAM), read only memories (ROM),electrically erasable programmable read-only memory (EEPROM), and thelike, can also be utilized to implement the exemplary computing systemand environment.

Any number of program modules can be stored on the hard disk 516,magnetic disk 520, optical disk 524, ROM 512, and/or RAM 510, includingby way of example, an operating system 526, one or more applicationprograms 528, other program modules 530, and program data 532. Each ofthe operating system 526, one or more application programs 528, otherprogram modules 530, and program data 932 (or some combination thereof)may include elements of the video encoding and/or decoding algorithmsand systems.

A user can enter commands and information into computer 502 via inputdevices such as a keyboard 534 and a pointing device 536 (e.g., a“mouse”). Other input devices 538 (not shown specifically) may include amicrophone, joystick, game pad, satellite dish, serial port, scanner,and/or the like. These and other input devices are connected to theprocessing unit 504 via input/output interfaces 540 that are coupled tothe system bus 508, but may be connected by other interface and busstructures, such as a parallel port, game port, or a universal serialbus (USB).

A monitor 542 or other type of display device can also be connected tothe system bus 508 via an interface, such as a video adapter 544. Inaddition to the monitor 542, other output peripheral devices can includecomponents such as speakers (not shown) and a printer 546 which can beconnected to computer 502 via the input/output interfaces 540.

Computer 502 can operate in a networked environment using logicalconnections to one or more remote computers, such as a remote computingdevice 548. By way of example, the remote computing device 548 can be apersonal computer, portable computer, a server, a router, a networkcomputer, a peer device or other common network node, and so on. Theremote computing device 548 is illustrated as a portable computer thatcan include many or all of the elements and features described hereinrelative to computer 502.

Logical connections between computer 502 and-the remote computer 548 aredepicted as a local area network (LAN) 550 and a general wide areanetwork (WAN) 552. Such networking environments are commonplace inoffices, enterprise-wide computer networks, intranets, and the Internet.

When implemented in a LAN networking environment, the computer 502 isconnected to a local network 550 via a network interface or adapter 554.When implemented in a WAN networking environment, the computer 502typically includes a modem 556 or other means for establishingcommunications over the wide network 552. The modem 556, which can beinternal or external to computer 502, can be connected to the system bus508 via the input/output interfaces 540 or other appropriate mechanisms.It is to be appreciated that the illustrated network connections areexemplary and that other means of establishing communication link(s)between the computers 502 and 548 can be employed.

In a networked environment, such as that illustrated with computingenvironment 500, program modules depicted relative to the computer 502,or portions thereof, may be stored in a remote memory storage device. Byway of example, remote application programs 558 reside on a memorydevice of remote computer 548. For purposes of illustration, applicationprograms and other executable program components such as the operatingsystem are illustrated herein as discrete blocks, although it isrecognized that such programs and components reside at various times indifferent storage components of the computing device 502, and areexecuted by the data processor(s) of the computer.

An implementation of the system and methods described herein may resultin the storage or transmission of data, instructions, or otherinformation across some form of computer readable media. Computerreadable media can be any available media that can be accessed by acomputer. By way of example, and not limitation, computer readable mediamay comprise “computer storage media” and “communications media.”“Computer storage media” include volatile and non-volatile, removableand non-removable media implemented in any method or technology forstorage of information such as computer readable instructions, datastructures, program modules, or other data. Computer storage mediaincludes, but is not limited to, RAM, ROM, EEPROM, flash memory or othermemory technology, CD-ROM, digital versatile disks (DVD) or otheroptical storage, magnetic cassettes, magnetic tape, magnetic diskstorage or other magnetic storage devices, or any other medium which canbe used to store the desired information and which can be accessed by acomputer.

“Communication media” typically embodies computer readable instructions,data structures, program modules, or other data—in a modulated datasignal, such as carrier wave or other transport mechanism. Communicationmedia also includes any information delivery media. The term “modulateddata signal” means a signal that has one or more of its characteristicsset or changed in such a manner as to encode information in the signal.By way of example, and not limitation, communication media includeswired media such as a wired network or direct-wired connection, andwireless media such as acoustic, RF, infrared, and other wireless media.Combinations of any of the above are also included within the scope ofcomputer readable media.

Alternatively, portions of the-systems and methods described herein maybe implemented in hardware or a combination of hardware, software,and/or firmware. For example, one or more application specificintegrated circuits (ASICs) or programmable logic devices (PLDs) couldbe designed or programmed to implement one or more portions of the videoencoding or video decoding systems and procedures.

Although the description above uses language that is specific tostructural features and/or methodological acts, it is to be understood-that the invention defined in the appended claims is not limited to thespecific features or acts described. Rather, the specific features andacts are disclosed as exemplary forms of implementing the invention.

The invention claimed is:
 1. One or more computer storage media storingcomputer-executable instructions for causing a computer systemprogrammed thereby to perform a method of indicating timing for pluralpictures in a video sequence, the one or more computer storage mediabeing selected from the group consisting of non-volatile memory,magnetic storage media and optical storage media, the method comprising:determining a first timestamp associated with a first picture of thevideo sequence; for the first timestamp, transmitting a parameter valuefor each of plural timestamp parameters, wherein the plural timestampparameters include an offset value that relates time associated with apicture to true time; determining a second timestamp associated with asecond picture of the video sequence; and for the second timestamp,transmitting a parameter value for each of one or more but not all ofthe plural timestamp parameters.
 2. The one or more computer storagemedia of claim 1 wherein the method further comprises: before thetransmitting for the first timestamp, transmitting a first flag valuethat designates the first timestamp as a full timestamp; and before thetransmitting for the second timestamp, transmitting a second flag valuethat designates the second timestamp as not being a full timestamp. 3.The one or more computer storage media of claim 1 wherein the methodfurther comprises: along with the transmitting for the second timestamp,transmitting a flag value for each of one or more flags that indicatewhich of the plural timestamp parameters have parameter valuestransmitted for the second timestamp.
 4. The one or more computerstorage media of claim 1 wherein the method further comprises:transmitting timebase information for the video sequence, wherein thetimebase information includes first timebase information and secondtimebase information, the first timebase information indicating a numberof clock units per second, and the second timebase informationindicating a number of clock units per picture.
 5. The one or morecomputer storage media of claim 1 wherein the plural timestampparameters further include a frame number parameter.
 6. The one or morecomputer storage media of claim 1 wherein the method further comprises:for each of one or more other pictures of the plural pictures in thevideo sequence: determining another timestamp associated with the otherpicture; and transmitting for the other picture a parameter value foreach of one or more of the plural timestamp parameters; wherein thedetermined timestamps are transmitted as full timestamps at an interval.7. The one or more computer storage media of claim 6 wherein theinterval is a regular interval of every X frames, X indicating a numberof frames.
 8. The one or more computer storage media of claim 6 whereinthe interval is a regular interval of several times per second.
 9. Theone or more computer storage media of claim 6 wherein the method furthercomprises: estimating reliability of a communication link; and adjustingthe interval based on the estimate.
 10. The one or more computer storagemedia of claim 1 wherein the plural timestamp parameters further includean hours parameter, a minutes parameter, a seconds parameter and a framenumber parameter, the method further comprising: determining, using aclock, a parameter value for each of the hours parameter, the minutesparameter and the seconds parameter; and determining, using a framecounter, a parameter value for the frame number parameter.
 11. One ormore computer storage media storing computer-executable instructions forcausing a computer system programmed thereby to perform a method ofdetermining timing for plural pictures in a video sequence, the one ormore computer storage media being selected from the group consisting ofnon-volatile memory, magnetic storage media and optical storage media,the method comprising: for a first timestamp associated with a firstpicture of the video sequence, receiving a parameter value for each ofplural timestamp parameters; determining the first timestamp based atleast in part on the plural received parameter values for the firsttimestamp; for a second timestamp, receiving a parameter value for eachof one or more but not all of the plural timestamp parameters, and,along with receiving the second timestamp, receiving a flag value foreach of one or more flags that indicate which of the plural timestampparameters have parameter values transmitted for the second timestamp;and determining the second timestamp based at least in part on the oneor more received parameter values for the second timestamp.
 12. The oneor more computer storage media of claim 11 wherein the method furthercomprises: maintaining at least part of the first timestamp for use inthe determining the second timestamp.
 13. The one or more computerstorage media of claim 11 wherein the method further comprises: beforethe receiving for the first timestamp, receiving a first flag value thatdesignates the first timestamp as a full timestamp; and before thereceiving for the second timestamp, receiving a second flag value thatdesignates the second timestamp as not being a full timestamp.
 14. Theone or more computer storage media of claim 11 wherein the methodfurther comprises: receiving timebase information for the videosequence, wherein the timebase information includes first timebaseinformation and second timebase information, the first timebaseinformation indicating a number of clock units per second, and thesecond timebase information indicating a number of clock units perpicture.
 15. The one or more computer storage media of claim 11 whereinthe plural timestamp parameters further include a frame numberparameter.
 16. One or more computer storage media storingcomputer-executable instructions for causing a computer systemprogrammed thereby to perform a method of indicating timing for pluralpictures in a video sequence, the one or more computer storage mediabeing selected from the group consisting of non-volatile memory,magnetic storage media and optical storage media, the method comprising:determining a first timestamp associated with a first picture of thevideo sequence; for the first timestamp, transmitting a parameter valuefor each of plural timestamp parameters; determining a second timestampassociated with a second picture of the video sequence; for the secondtimestamp, transmitting a parameter value for each of one or more butnot all of the plural timestamp parameters; for each of one or moreother pictures of the plural pictures in the video sequence: determininganother timestamp associated with the other picture; and transmittingfor the other picture a parameter value for each of one or more of theplural timestamp parameters; wherein the determined timestamps aretransmitted as full timestamps at a regular interval.
 17. The one ormore computer storage media of claim 16 wherein the plural timestampparameters include an hours parameter, a minutes parameter, a secondsparameter and a frame number parameter, the method further comprising:determining, using a clock, a parameter value for each of the hoursparameter, the minutes parameter and the seconds parameter; anddetermining, using a frame counter, a parameter value for the framenumber parameter.
 18. The one or more computer storage media of claim 16wherein the regular interval is every X frames, X indicating a number offrames.
 19. The one or more computer storage media of claim 16 whereinthe regular interval is several times per second.
 20. The one or morecomputer storage media of claim 16 wherein the method further comprises:estimating reliability of a communication link; and adjusting theregular interval based on the estimate.